This invention relates to an expander spring for piston rings, particularly for lamina-shaped oil scraper rings and is formed of a cross-sectionally approximately U-shaped, annularly bent sheet metal strip which is rendered resilient by circumferentially distributed slots or similar cutouts and which, in the zone of at least one radial side, has axially projecting support feet for engaging at least one oil scraper ring.
For the radial biasing of oil scraper piston rings which are generally made of a bent steel strip, there are known a substantial mumber of expander spring structures which are formed of sheet metal strips of various cross-sectional configuration and are made resilient in the circumferential direction by means of slots or the like. Expander springs which are particularly easy to manufacture have an L or U-shaped cross section. Expander springs of L-shaped cross section are disclosed, for example, in U.S. Pat. No. 3,627,333 which have, when viewed axially, a very small height, so that upon application of a radial tensioning force, the spring undergoes significant distortion. Further, in such expander springs there are provided no oil discharge openings so that they are practically not adapted to be used with oil scraper rings. In contradistinction, expander springs having a U-shaped cross section are more advantageous in these respects. Particularly, for manufacturing reasons, the cross-sectional profile of expander springs has been arranged in such a manner that the U profile is open either outwardly towards the wall of the cylinder in which the piston works or inwardly towards the piston, as disclosed, for example, in U.S. Pat. No. 2,837,385 and German Pat. No. 1,475,713.
Expander springs of the above-outlined type serve not only for the radial biasing of the oil scraper rings, but they also must guide the oil scraper rings which, in most cases, are very thin in the axial direction. Thus, the expander springs must prevent tilting of the oil scraper rings in the gap between the expander spring and the piston.
Expander springs with U-shaped cross section do not fully satisfy the above requirement, since the free leg ends of the "U" do not have sufficient guiding properties. Although the expander spring disclosed in U.S. Pat. No. 3,191,947 has additional axial extensions at one of the leg ends for preventing an axial bending of the two leg ends of the "U" to one another, an expander spring having a box-like cross section resulting from such a structure is difficult to manufacture. Further, the lower leg itself of the expander spring is designed as the oil scraper and thus is exposed to a high degree of wear.